Depending on where you are located, maybe your big box store may have them, if it is not snowy cold there..or online, ebay, but I see only seeds right now. Or some Florida nurseries. I also saw a seller in Amazon.com, not sure though if it is a grafted one or from seed.

Yellow is still a pretty unusual color for Desert Roses so they are not as commonly available as the other colors. I personally would not depend too much on a claim of 'yellow' but only purchase one that you can see in bloom, to avoid disappointment.
Here is a double yellow one I bought two years ago. Paid big bucks...

Sorry, I don't propagate Adeniums but check out Florida Colors in Homestead. In addition to Plumeria (their main focus) they also sell Adeniums and are about to launch their range on their website. Meanwhile you can give them a call and discus what you'd like.